From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from pigeon.gentoo.org ([208.92.234.80] helo=lists.gentoo.org) by finch.gentoo.org with esmtp (Exim 4.60) (envelope-from ) id 1Q97wU-0006LY-KI for garchives@archives.gentoo.org; Mon, 11 Apr 2011 03:37:34 +0000 Received: from pigeon.gentoo.org (localhost [127.0.0.1]) by pigeon.gentoo.org (Postfix) with SMTP id CB8911C005; Mon, 11 Apr 2011 03:35:50 +0000 (UTC) Received: from smtpout.karoo.kcom.com (smtpout.karoo.kcom.com [212.50.160.34]) by pigeon.gentoo.org (Postfix) with ESMTP id 6F2F11C005 for ; Mon, 11 Apr 2011 03:35:50 +0000 (UTC) X-IronPort-AV: E=Sophos;i="4.63,336,1299456000"; d="scan'208";a="500136751" Received: from 213-152-39-90.dsl.eclipse.net.uk (HELO compaq.stroller.uk.eu.org) ([213.152.39.90]) by smtpout.karoo.kcom.com with ESMTP; 11 Apr 2011 04:35:49 +0100 Received: from [192.168.1.100] (unknown [192.168.1.100]) by compaq.stroller.uk.eu.org (Postfix) with ESMTP id C6EFDA93BD for ; Mon, 11 Apr 2011 04:35:47 +0100 (BST) Content-Type: text/plain; charset=us-ascii Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-Id: Gentoo Linux mail X-BeenThere: gentoo-user@lists.gentoo.org Reply-to: gentoo-user@lists.gentoo.org Mime-Version: 1.0 (Apple Message framework v1084) Subject: Re: [gentoo-user] [OT] Disk recommendations? From: Stroller In-Reply-To: Date: Mon, 11 Apr 2011 04:35:43 +0100 Content-Transfer-Encoding: quoted-printable Message-Id: <636A1DAA-F889-46CB-BB66-D39109E58854@stellar.eclipse.co.uk> References: <201104092100.19783.peter@humphrey.ukfsn.org> <201104100850.41768.peter@humphrey.ukfsn.org> To: gentoo-user@lists.gentoo.org X-Mailer: Apple Mail (2.1084) X-Archives-Salt: X-Archives-Hash: 40a933ec2443bd7a53af3a8a056bc707 On 10/4/2011, at 2:50pm, Mark Knecht wrote: > ... loses 1 drive and > then, while in the process of fixing the RAID, loses a second drive. > Most of us (myself included) buy identical drives all at the same time > from the same vendor. This means all the drives were likely from the > same manufacturing batch and, if they are drives that will fail at all > then the group will likely experience multiple drive failures. It doesn't make it *likely* that they'll fail simultaneously. It makes = it less unlikely. > The > underlying idea of RAID is that the drives are not likely to fail at > the same time giving us time to fix the array. However, if /dev/sda > fails the chances of /dev/sdb failing is higher if they were built at > the same time in the same plant. ^ This is a more accurate synopsis.=20 > Reading the mdadm list for the last couple of years it seems that many > folks running data centers intentionally buy drives from multiple > manufactures, or drives of different sizes from the same manufacturer, > hoping to lower the chances of multiple failures at the same time. I've found it sometimes quite inconvenient to do this, and whilst I = consider it good practice I get the impression a lot of people, perhaps = the majority, don't bother (or don't even know they should). I kinda = think it's a nice thing to do but not essential - I don't know that the = risk of simultaneous failure is increased that significantly. Many = high-end servers will be sold off-the-shelf by their manufacturers with = consecutively-serialed drives in the RAID array - I don't think this is = considered risky enough for Dell or IBM to offer non-matching drives as = a purchasing option. One also has to wonder what the performance implications might be of = having three drives in an array with slightly different rotational = speeds, spin-up and seek times. Ultimately, we shouldn't be fully dependent upon RAID for the integrity = of our data, anyway. "RAID is not a backup" is the famous saying. =20 > As for hardware RAID the risk I hear about there is that if the > controller itself fails then you need an identical backup controller > or you risk the possibility that you won't be able to recover > anything. I don't know how true that is or whether it's just FUD. Generally you just need a similar one. In the case of 3ware you can connect your drives to any other 3ware = controller and it will recognise the array descriptors written at the = start of the drive. I haven't swapped drives between the PERCs (rebadged Adaptec, I think) = of Dell 2650s & 2850s, but these machines are now so cheap on the = secondhand market anyway, you can afford to have a spare identical one. I think you're over-estimating the *risk* of being unable to find a RAID = controller of the same model. But certainly if you buy a good PCIe SATA = card on the secondhand market it will not be cheap to replace in the = event of failure, and a bargain may not come up on eBay immediately. So = I think you'll certainly be able to recover your data, you may just some = inconvenience of having to wait to find a cheap enough card or spend a = lot of money buying an obsolete card in a hurry. Ideally, you have a = spare in advance or buy hardware RAID under a 5-year warranty (in which = case it's replaced next-day by the manufacturer). This is really a matter of horses-for-courses. Most people (including = myself) don't really "need" hardware RAID. Hardware RAID is much more = expensive, but I do consider it "better", if only because you can = hot-swap. That is not assured with cheap SATA controllers. OTOH Linux's software RAID does seem to be just as fast (??) as hardware = RAID, and has some cool features. Stroller.